Pure & Appi. Chem., Vol. 56, No. 3, pp. 403—416, 1984. 0033—4545/84 $3.OO+O.OO

Printed in Great Britain.Pergamon Press Ltd.

THERMOTROPIC LIQUID CRYSTALLINE POLYMERS IN ELECTRIC AND

MAGNETIC FIELDS

N.A.P1at, R.V.Talroze, V.P.Shibaev

I1ymer Chemistry Department Moscow State University,Moscow, USSR

Abstract — A be)aaviour o± therraotropic liquid crystallinepoTyiners in the electric and magnetic ±ields is discussed.Experimental data are given on the orientation and relaxa-.tion processes, Frideriks effect, "guest—host effect",influence of molecular mass on the electrooptical proper—ties of some LO polymers. Phenomena of electrohydxo&ynamicinstability in such polytners are discussed, new structuraltransition induced by electric field is discovered, prim-.ciples of therxnoadressing and information recording usingLC polymers are described.

INTRODUCTION

It is well known now, that polymers having mesogenic groups in backbonesand in side branches of macromolecules can exist in the state equivalentfollowing many features to the liquid crystalline (LC) state of low mole.-cular compounds (Ref .1—5).

One of the significant proofs for LC state in polymers in their capabili-ty to orient when external electric and magnetic fields are applied. Thisis the quality of low molecular LC which provided so wide applicationsof these materials in electrooptic devices. Electro— and. magnetoopticsof LC is now well developct field of physics and technics.

As to electro-. and. magnetooptics of polymeric LC first publicationsappeared only 2—3 years ago. Here we shall give a first review of quali.-tative picture of phenomena? observed with the analysis of similarityand differences in the behaviour of polymeric and low molecular LO inthe external fields. Main attention will be paid to the researches carri-ed out in our laboratory at the University of Moscow in 1980—1983,

Let us remind the main types of electro— and magnetooptic phenomenaknown for low molecular LC and first of all for nematics.

The most of electro— and magnetooptical effects are related to the reori-entation of the director (or optical axis) of the macroscopic volume ofthe liquid crystal under the influence of the field or of the liquid'sflow. The orientation is caused by the anisotropy of electrical and mag-.netical properties of liquid crystals such as dielectric and diamagneticsusceptibility and electric conductivity. Besides that the process ofrearrangement depends on the initial orientation of a liquid crystal and.its visco—elastic properties. The change of optical properties as aresult of the reorientation process is already the consequence of theoptical anisotropy of liquid crystals.

All known electro— and magnetooptical phenomena can be divided into twomain groups: field effects when deformation of liquid crystals by theexternal field is not related with the electric conductivity — andelectrohydrodynarnical instabilities (EHD), which are caused by the fluid'sflow induced by the electric field. EHD instabilities occur in liquidcrystals when impurity caused electric conductivity is high enough.

403

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0BIEfliDVJ0I1 LHE1I0WEMV II I LOLX.WEI2

V E3 cI

Thermotropic liquid crystalline polymers 405

TABLE 1. Mesophase type, glass (Tg) and clearing (T01)temperatures of some nitrile—containing LO

polymers of comb—like structure

E dH2-O(Y)

**000 —

(OH2)m_ 0—O6114—000—O6H4—Xla OH, 6 60

0H 6 O11AOOH** ) U9

lb CH 6 600H 2 0C11A°011** -, v'pic OH 2 70OH-I 6 OH OCH

** ) OLFid OH 2 —

OH32

O6H400H32a H 2 ON 62

2b H 6 ON 25

2c H 2 N=OH—O6H4ON 72

2d H 6 N=OH—C H ON 35

.[ OH-O(Y)-COO - (OH)1 0—O6H4—O6H4—CN

3a0113

2 95

3b OH3 5 60

3c0113

ii 40

4a H 2 50

4b H 5 40

4c H ii 25

{ OH2-9(J-000 —

(OH2)ffi— 0—O6H4—OH=N—O6H4—ON6 355a OH3

5b0113

6a H6b H 11

5— smectics*

N— nematics

Pflflfllvmcvc

it means the orientation of the side mesogenic groups.

The orientation induced by an electric field can be fixed on cooling thepolymer below T • Such polymer film is characterized by the conoscopicfigure presente in Pig.iC. It shows the optical equivalency of this sys-tem to an anisotropic uniaxial crystal with the axis direction coincidingwith the electric field direction.

Thus a nematic polymer of comb—like structure can be oriented in an elec-tric field like a low molecular nematic, in spite of the chemical bindingof the mesogenic groups to the main chain.

No*

Polymers Tg0OT 00 Mesophase typecl'Y m

213

227

195 N

224 N

93 N109 N267 N211 N

121 S

121 S

112 N

120 N145 S

i25 S

155 5

158 N

169 5

ii

6

25

20

10

406 N. A. PLATE et al.

The duration of the orientation process essentially depends on the poly.-merization degree .P enhancing with its increase (Fig.2) (Ref.17 & 18).

100

50

4

Pig. 2. (a) Optical transmittance as a function of time atthe voltage U = IOOV (f = 50 H) for the polymer 4b havingpolymerization degree P: 20 (1), 70 (2), 240 (3), 1200 (4)(film thickness d = 12mm, T = Ti Te = 3O)(b) Rise-.time of Frederics effect as a function of intrinsicviscosity of polymer 4b (U = 100 V, d. = l2jtm, AT = T—T c

exp

The reveree process of the relaxation is also dependent on P. In(Ref.16) the relaxation have not been already observed in polymers with

of about 70 and higher. At the sane time the values of rise—time andrelaxation time, which have been reported by Ringsdorf et al. and givenin the Table 2 for some ON—containing acrylic polymers, are coniraensurablewith the kinetic characteristics of low molecular nematics. This factpermitted to predict the perspective of the usage of polymer liquid crys-tal as high speed responsable subjects. It can be supposed, however, thatlow rise and. relaxation times observed in (Ref.11) indicate the extremelylow polymerization degrees of polymers studied. Therefore the correct coin—parison of the properties of high and low molecular liquid crystals needsthe exact indication of the molar mass of the corresponding polymer.

The orientation time depends on the temperature (Fig.3) (Ref.27). Thestudy of the kinetics, which has been carried out in (Ref.19) for polymericcyandiphenyl and azomethyne derivatives, has shown, that the curves of thechange of the optical transparency (Fig.3) are related to the field inclu-.ced. relaxation process and can be described by the equation given below:kT

1— &where f9 degree of completness of the process; D time; k and rL-constants (1 I in the wide temperature range and it doesn't depend onthe polymerization degree).

The constant k which can be considered as a rate constant of the orienta-.tion process exponentially depends on the temperature. That permits tocalculate the effective activation energy of the orientation processwhich is equal to 80—200 1Q /mol (Table 3). Any physical model of theorientation of the comb—like LC polymer corresponds to the real mechanismif it explains two experimental facts: the essential dependence of theorientation time on the main chain length (P) and the slight influenceof the polymerization degree on the activation energy value for each poly-mer given. The latter is a strong argument in favour of the common riecha-.nism of the orientation process in the range of molecular masses studied.

mm

8

6

4

2

2 4 0,1 0,2 dl/g

Thermotropic liquid crystalline polymers 407

TABlE 2. Threshold voltage (IJo), rise (1) and relaxationtime of some comb—like LC polymers (Ref.11)

COO•'(CH2)_O_C6H4C00_C6H4-X

TexpTg U0 ,V S

CA' 2 22 60 14 87

CJV 6 76 5 3 3

NC/1C1-IyCJ/ 2 84 23 3 3

MC/4C6/4qCJY 6 120 4 1 4

layer thickness - 2011A-m, Texp — temperature of the experiment

TABLE 3. Activation energy (BA) and rise—time '1/2 of theorientation for polymeric azomethyne derivatives of diff e—rent polymerization degree

f2-riCOO_(CH2)_O C6H, -CH=N— C6NT ON

PtDlymer in Mesophase '1/2' , E,ype kj/mol

H 6 N 5—10

250 18 97

350 25 105

1200 510 105

H 11 S 10 84

H (80 mole %) 1 6 S 120 170

OH3 (20 mole 11

The value of the viscous flow activation energy (BA 70—90 kj/mol) of theneinatic comb—like polymer 4b in the LC phase is close to BA values givenin the Table 3. One can suppose that the segments of macromolecules areinvolved in the orientational movement. The enhancing of the orientationtime with the increase of the polymerization degree means, that the posi-tion of macromolecule as a whole is changed. But the process itself iscaused by the tendency to orientation of side chains which form the LCphase with the positive value of iE

The activation energy of the orientation of the acrylic comb—like polymersof snietic structure is about the same value as in nematic systems (Table 3).This fact is not typical for low molecular sinectics which are oriented

more difficult in comparison with nematics. That is apparently relatedto the polymeric nature of substances studied. BA depends not on the meso—phase structure but on the mobility of main chains taking part in theorientational process.

408 N. A. PLATE et al.

I,%100

50

Fig. 3. Change of transparency of nitrile containing comb—like polymers during the orientation process at differenttemperatures: T1(1) > T2(2) > T(3) '1/2 time of 50%change of optical transmittance).

The increase o± the orientation activation energy in copolymer with methac—rylic units (Table 3) and thedelay of the orientation in methacrylic homo—polymers (Ref.19) confirm the decisive role of the main chains mobilityand proves the participation of the main chain in the process of the orien—tation of comb—like polymer in an electric field.

Studying polymeric and low molecular liquid crystals from the viewpointof the analogy in their electrooptical behaviour such useful characteris-tic of Frederics effect as threshold voltage must be considered. The useof some effective value of the threshold voltage (Ref.8) permits to estab-lish some qualitative regularities. Taking for threshold voltage U thevoltage at which no change of the optical transmittance during at least15 mm was observed we have studied the frequency dependence of U : == (f) (Fig.4). The results obtained (Fig.4) are in a good agreement withthe dependence of the threshold voltage of Frederics effect on the frequen-cy of the electric field well—known for low molecular nematics.

The data given in Fig.4 show also the existance of the frequency correspon-ding to aE = 0 at which the value U tends to infinity. Above this frequ-ency for the dielectric anisotropy changes its sign as a result of thepart of the orientational polarization. That is also confirmed by the in-crease of the relaxation process rate in the electric field of high frequ-ency (Fig.5). After the field homeotropic orientation starts to relax(curve 2). At the repeated applying of the field (at 't = 135 s) of increa-sing frequancy (curves 3—6) the rate of orientation is going down(curves 3, 4). At frequencies above the critical value (in this case —6 kHz) reorientation occurs, which is induced by the turn of niesogenicgroup in the direction normal to the field direction (curves 5., 6). Thiseffect can be ecplained by the sign change of AE — effect known for lowmolecular nematics with a positive , having a perpendicular componentof the dipole moment. In this case specific features of the polymer aremanifested themselves by the decrease of the 1 sign change frequency.That can be related to the high viscosity of polymers. Beside this due tokinetic restrictions the orientation, which does not correspond to the Asign at given temperature can be fixed in the polymer by its cooling below

Tg•

"GUEST—HOST EFFECT"

The "guest—host" effect which is known in low molecular liquid crystalsis detected also in polymers. The host role is played by LC polymer matrixand the pleochroic &ye is a guest. Dye molecules are characterized by an

3

II

I I

I I,31/2 '1/2 20

I I

40 60

Thermotropic liquid crystalline polymers 409

20 4O 60$ =0 E

Pig. 5. Optical transmittanceas a function time upon appli.-cation of an electric field(U = 80V, T = 2°) of diff e—rent frequency 50 Hz (1), 1kHz(3), 5kHz (4), 7kHz (5), 20kHz(6) and relaxation uponswitching the field off (2).

elongated form with the absorbtion oscillator, directed along the longmolecular axis. Dye can be incorporated in polymer either mechanically(Ref .21 ,22) or by chemical binding of guest molecules with polymer matrix(Ref .23).

The incorporation of 1—2% by weight of the low molecular azo-.&yestoff(Ref .21)

02N- -N=N-. -N 02H5

C2H4OH

or of stylbene—derivative (Ref .22)

02N- -CH=CH- -N < 3

has been shown not to change polymer mesophase structure. Electric fieldinduces an alignment of a guest dye (Fig.6) in a host polymer which resultsin a change of colour of the polymer film as a function of the dye type,A C sign and the external field parameters • The experimental data givenin (Ref.21—23) indicate the possibility of the creation of indicators withcontrolled colour characteristics.

"Guest—host" effect can be used for the determination of the order parame-ter S of the liquid crystal. The values of S estimated by comparative ab—sorbtion measurements of comb—like nematic polymers with low moleculardyes are about 0.3—0.5 (Ref.21 & 22).

Thus the compatible compositions of LC comb—like polymers with the pleo—chroic dye stoff can be obtained. This fact in combination with the "guest—host" effect makes possible to vary the colour characteristics of polymericfilms and to obtain quantitive information about the orientational orderof the IC polymer.

Uov

401

I,

30

6

20

10

.5

1

2

4

io1 io2 -Io 10 f,HzPig. 4. Threshold voltage asa function of the electricfield frequency for the poly-mer 4b: T 18°(1),13°(2),90(3)

3

8

410 N. A. PLATE et al.

INFLUTCE OP MAGNETIC FIELD ON LC POLYIVIERS

As it has been alrea&y mentioned above the frederics effect can be inducedboth by electric and magnetic fie1 Due to the anisotropy of the diamag-netic susceptibility the LC polymers as well as low molecular liquid crystals are able to be oriented in magnetic field. This fact has been demon—strated in NR experiments with comb—like polymers containing deuterosub—stituted phenyl esters of p—bydroxybensoic acid as rnesogenic groups andwith cyandiphenyl—containin polymers (Ref .25) . The latter have been syn—thesized and studied in our laboratory. The orientation in magnetic fieldhas been shown by proton magnetic resonance ('ENMR) experiment and by X—ray analysis. The transition from the isotropic melt to the LO phase isaccompanied by the change of the narrow singlet (Fig.7a) into a partiallyresolved triplet, which is typical for oriented liquid crystal (Fig.7b).

cuiiii iiiii i ii ii ii it tijillib1/2 /I/// I \1\/) t"— /\\ I '/ " / QkHz

cll111F1 I(ItIIIIItLlI l1IIIlL1i11ita)

c_II1-IIIIIIIIIIIII(ILII(111I1t11111113J

I o 01

II IIIiIi?iIoIITIIT1111L (LIII 111111111 tl I1LIi]1D

b)

Fig. 6. "Guest—host" effect Fig. 7' 1}fl spectra o± thein an electric field U = 0 polymer 4b at different tern—(a) and U 0 (b): I — meso— peratures: in isotropic meltgenic groups, 2 — dye mole— (a) 1200 and in LO phase,cules. 80° (b).

As well as in the case of electric field the orientation of side niesogenicgroups along the magnetic field direction has been confirmed by X—ray data.

The order parameter S can be calculated from the splitting constant of sidecomponents of a partially resolved triplet which arise from a direct dipo-.le—dipole interaction of the benzene ring protons.

The value S = 0.45 is in a good agreement with the S values which are ob-tained by other methods (Ref.21—23,26).

if the orientation in electric field has been observed only for polymersof comb—like structure, the magnetic field induces the orientation oflinear polymers having mesogenic groups in main chains also (Ref.27—29).

Studying the orientation properties of linear polymers in external fieldsthe question arises: how many units are included in a IC domain, whichis oriented in the field ? That is not clear yet, so the oligomer fractionconsisting of 10—20 and even 30 monomeric units can't be regarded as equi-valent model of polymer chains • It is possible, that the transition to po-lymers with higher polymerization degrees can be accompanied by the changeof the mechanism of orientation in magnetic field.

EIECTROHYDRODYNAMICAL INSTABILITIES IN IC POItvERS

The second group of effects is related with the appearance of electrobydro—dynamical (HD) instabilities in polymers. The formation of instubilitypatterns such as Kapustin—Williams domains in two different linear polymers:

1qiz

Thermotropic liquid crystalline polymers 4fl

copolyester of terephtalic and. hydroxybensoic acids with etylene glyco].

0220]- '-and polyester synthesized from —raetbylstilbene and adi.-

pic acid.

fCo.(cH2)4COO_ 2J_H... }.

has been reported by Krigbaum et al.(Ref.30—32). This fact indicates thatthese polymers form the C phase of neniatic structure with negative dielec—trio anisotropy (LXE . 0) iid positive conductivity anisotropy (i7> 0).Unlike low molecular liquid crystals the times of the domain structureformation in IC polymers are about many tenths of minutes. That is undoubt—ly related to the high viscosity of polymer melts.

As to comb—like nitrile—containing polymers the formation of domains inthese systems has not been observed as it could be expected in polymerswith high positive 6 • At the same time an applying a.c. electric fieldto homeotropic oriented f1ms. of polymers

.CH2-CI4

CoO(CH2)-O- -CH=N-

where in = 6.11

induces the intensive 'boiling° of LC melts resulted in the sharp distor-tion of the optical transparency (Ref.18 & 19). That seems to be an ERDprocess of dynamic scattering mode (IBM) type. Data shown in Fig.8 indica-te that the appearance of LSM in polymer liquid crystals essentiallydepends on the temperature. With the increase of the electric field frequ-ency the ]M region becomes narrower: the lower limiting temperatureTEHD is shifted to the higher temperatures (at higher frequency the increa-sed ions mobility for flow of liquid is needed).

T,°C

120

80

1000 f,Hz

Fig. 8. Temperature of the appearence of EHD instability inhomeotropically oriented films of polymers 6a(1) and 6b(2)as a function of the electric field frequency (U = 200 V,d = 12Jm)

At lower temperature in the high frequency field the ions oscillate onlyand that doesn't prevent the homeotropic orientation.

Such type of the frequency dependence of TERD (Fig.8) permits to realizethe two—frequency switching of opticl characteristics of polymer fjlrns.At the given temperature T (where TD (T <T4D) one can change the

2

1

200 600

412 N. A. PLATE et al.

regime of homeotropic orientation to an electrohydrodynamic one by varia.-tion of the electric field frequency (Ref.19) (Fig.9).

I,%

100

50

11 ,$

Fig. 9. KinetIcs of the change of the transparency of poly.-iners 6a(1) and 6b(2) at the discrete switching oI thefield frequency (U = 250 V, d = 12 .M.m, T = 125°C; 14OO Hz(Ia), 14 kHz (Ib), 200 Hz (2a) and. I kHz (2b)

By switching on the low frequency field the initial honieotropio orientedfilm with the high transparency starts to scatter the light due to EHDinstability. The transparency is 5—6 times decreased. The use-time of DSMin polymers exceeds the time of the orientation process and depends on thefield frequency. As in linear systems DSM process in comb—like polymersmanifests itself at high temperature at which the viscosity the LC meltis not too high • If in the polymer with P 100—200 EHD process takesplace in the temperature rabge wide enough, DSM in the polymer with P1500 in an electric field (at ±' = 50 Hz) appears only several degreesbelow Tci and manifests itself very poorly.

Jmain structure formation in linear polymers can be explained in theframework of the classical Carr—Helfrich.-.Williams mechanism (Ref .32 & 33),In nitrile—containing polymer Shiff's bases ionic current impurity conduc-tivity results in LM effect. As far as the value of L & is positive andhigh enough, the EHD instability in CN—containing polymers can't be des-cribed with the mechanism mentioned above. The process seems to have theisotropic mechanism typical for isotropic liquid.s (Ref.311.).

COOPERATIVE STRUCTURE TRANSITION IN LC POfl(2RS INDUCED

BY THE ORIENTATION IN EXTERNAL FIELD

In this pare of the paper one peculiar effect observed for the first timeand unknown for low molecular analogues is discussed (Ref.35 & 36). Thehomeotropic oriented structure of some IC comb—like polymers has beenshown not to be fixed on cooling below T5. Being cooled the polymer filmoriented in an electric field undergoes stepwise change of the orientationwhich results in the sharp appearenee of the birefringence. As an examplethe dependence of the optical transmittance measured in crossed polarizersversus temperature for the polymer 6a (Table 1) is given in the Fig.1O.The study of the structure and dielectric characteristics have shown thatthe observed change of optical properties is related with the cooperativestructure transition. This transition also occurs both in electric fieldand when the electric field is switched off at the temperature 10—15° abovethe transition temperature. The transition takes place only in preliminaryoriented polymer, and the field creates this orientation, The transitiontemperature decreases with the increase of the field intensity (Fig.11).Such dependence means, that the homeotropic structure is stabilized by thefield, and the transition itself is caused by the "internal", thermodyna—

2

2

2 4 6 8 10 20 30

Thermotropic liquid crystalline polymers 413

100

50

50 fT 150T T,°0 200 TJ,V

Fig. 10. ange o± optca1 Pig. 11. Temperature of thetransmittance of the polymer structure transition as a func.6a with the temperature tion o± electric field voltage(U = 200 V, d = 12 J-m, for the polymer 6a (1) and. itscrossed polarizers) copolynier with methyirnethacry-.

late (2)

mical properties of the polymer. This effect is observed only. in polymersof nematic structure, in sinectic polymers the homeotropic orientation isknown to be stabilized below Tg

The detailed molecular mechanism of the effect mentioned above is not yet// clear. One can suppose that the transition is related with the change

of the conformational state of main chain in sites of the attachment ofside mesogenic groups oriented In an electric field.

THERMORECORDING ON LC BDLYMERS

Discussing the possibilities of practical use of electro—optical effectsin polymeric liquid crystals, it can be concluded that these systemscan't compete with low molecular ones in cases where the high speed ofresponse is needed. At the same time the polymeric state of liquid crys-tals leads to the creation of new optical materials with properties whichcan be regulated with the aid of external field in LC phase and thencan be fixed In the glass state.

This has been tested for the registration and the optical reflection ofinformation. At the first time the usage of LO polymers for this purposehas been reported by the team of researcher from Moscow State Uliiversity.

The principal scheme of the recording of information on theoriented layer of the LC polymer is given In Fig .12 (a—c).

On the transparent film of the homeotropially oriented LO polymer (Fig,12a) the regions of local overheating are created with the aid of laserbeam. In these local regions the liquid crystal passes over to an isotro-pic melt and the homeotropic orientation is destroyed (Fig.12b). Insteadof transparent monodomain homeotropic texture a polydomain texture thatscatters light is formed which stays on cooling (Fig.12c). In such a waysome information can be recorded on the transparent film and may be"wlpped off" by an electric field applying. Such type of recording isusually called as thermorecordin or therrrioadressing. For thermorecordingit is necessary for an oriented state to be stable in the absence of anelectric field for a rather long time, i.e. the orientation relaxationrate should be sufficiently low. The low molecular nematics don't satisfythis requirement. As In the smetic state the homeotroplc texture isstable enough the "smectic nematic" transition is mainly used inthermorecording low molecular LC devices.

PHAC 56:3—H

m 0tr'

82

78

74

70 21

100

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Thermotropic liquid crystalline polymers 415

differ beneficially from low molecular liquid crystals because the timeo± the recorded iifforrnation keeping in devices with low molecular liquidcrystals is most cases is limited by several days only. All this showsthe possibilities of the usage o± the LC state for the active controlover the structural—optical properties o± polymer materials.

CONCLUSIONS

Thus, the behaviour of polymeric and. low molecular liquid crystals hasmany common features. It must be stressed out that this similarity israther qualitative. The quantitative characteristics are differed strong-.ly : in polymers one can find the sharp increase of the time of' all pro cesSes and it ' s dependence on the mo lecular mass the strong temperaturedependence of the orientation time caused by the high values o±' activa-.tion energy.Results obtained during the short time from the very begimning of electro—optical study demonstrate the examples of new properties of polymericliquid crystals: the possibility of the freezing of oriented structureand it's stabilization in the absence of the field due to the glass tran—sition and high relaxation times within the IC pimse.The study of electro— and magnetoopical properties of polymers startsonly. The possibilities oi the searching and stu&y in this field are notyet completed. At the sane time the well known regularities of the beha—viour low molecular liquid crystals are a good basis for the stu&y oi morecomplicated polymer systems. One of the main task Thr future is the crea-.tion of physical model of polymeric LC which embraces the role of thebackbone in the conditions of external fields action and serves the back-.ground for the "continuity" of oriented polymeric LC phase. It should beimportant to know also to what degree the apparatus developped for lowmolecular IC is good enough for quantitative description of polymeric IC.

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